Use of interfacial condensation to encapsulate substances such as pharmaceuticals, pesticides and herbicides is discussed in U.S. Pat. No. 3,577,515, issued on May 4, 1971. The encapsulation process involves two immiscible liquid phases, one being dispersed in the other by agitation, and the subsequent polymerization of monomers from each phase at the interface between the bulk (continuous) phase, and the dispersed droplets. The immiscible liquids are typically water and an organic solvent. Polyurethanes and polyureas are included in the types of materials suitable for producing the microcapsules. The use of emulsifying agents (also known as suspending or dispersing agents) is also discussed. The United States patent discloses formation of microcapsules comprising a polymeric sphere and a liquid centre, ranging from 30 micron to 2 mm in diameter, depending on monomers and solvents used.
United Kingdom Patent No. 1,371,179 discloses the preparation of polyurea capsules for containing dyes, inks, chemical reagents, pharmaceuticals, flavouring materials, fungicides, bactericides and pesticides such as herbicides and insecticides. The capsules are prepared from various di- and polyisocyanates in a dispersed organic phase. Some of the isocyanate present reacts to yield an amine which reacts further with remaining isocyanate at the interface with water and subsequently polymerizes to form a polyurea shell. The aqueous phase also contains a surfactant, for example an ethoxylated nonylphenol or a polyethylene glycol ether of a linear alcohol. In addition, the aqueous phase contains protective colloids, typically polyacrylates, methylcellulose and PVA. Particle sizes as low as 1 micron are exemplified. Encapsulation of insect hormones and mimics are among the systems mentioned.
U.S. Pat. No. 4,046,741 and U.S. Pat. No. 4,140,516 appear to relate to developments of the process disclosed in United Kingdom Patent No. 1,371,179. According to U.S. Pat. No. 4,046,741, a problem with microcapsules is instability caused by evolution of carbon dioxide from residual isocyanate entrapped in the microcapsules. U.S. Pat. No. 4,046,741 discloses a post-treatment of polyurea microcapsules with ammonia or an amine such as diethylamine. This removes the residual isocyanate, allowing subsequent storage of the microcapsules at lower pH's without generation of carbon dioxide. U.S. Pat. No. 4,140,516 discloses the use of quaternary salts as phase transfer catalysts to speed up the formation of polyurea microcapsules.
U.S. Pat. No. 4,417,916 discloses encapsulation of water-immiscible materials such as herbicides in a polyurea shell. A polyisocyanate and a polyamine are used to form the polyurea, and the invention appears to reside in the use of a lignin sulfonate compound as emulsifier in the polyurea-forming reaction. The concentration range of water-immiscible material encapsulated in the examples listed is 320 to 520 g/L of composition.
U.S. Pat. No. 4,563,212 is similar in teaching to U.S. Pat. No. 4,417,916, but uses emulsifiers other than lignin sulfonates, particularly sulfonated naphthalene formaldehyde condensates and sulfonated polystyrenes.
European Patent No. 611 253 describes reaction of polyisocyanates and polyamines to encapsulate materials such as pesticides in polyurea, using non-ionic surfactants that are block copolymers containing hydrophillic blocks together with hydrophobic blocks.
Canadian Patent No. 1,044,134 is concerned with microencapsulation of insecticides, particularly pyrethroids. The insecticide is dissolved, together with a polyisocyanate, in a water-immiscible organic solvent. The solution in organic solvent is then dispersed in water by agitation, and a polyfunctional amine is added while agitation is continued. The polyisocyanate and the polyfunctional amine react to form a polyurea shell wall that surrounds the dispersed droplets containing the insecticide.
Canadian Patent No. 1,179,682 discusses encapsulation of pheromones. Microcapsules containing pheromones are produced from toluene diisocyanate and ethylene diamine and/or diethylene triamine. In one described embodiment, a polyamine in the form of a salt is added to a dispersion of isocyanate, to allow polymerization to be initiated by addition of a base. It is said that this may improve the stabilization of behaviour modifying compounds that are aldehydes, but this is not exemplified. Canadian Patent No. 1,179,682 states that pheromones are photolabile and lose efficacy after exposure to sunlight. A tertiary phenylene diamine is used as a light stabilizer, and is added with the water-immiscible phase so that it is ultimately encapsulated with the pheromone.